Method for inhibiting the oxidation of readily oxidizable metals



Patented Sept. 4, 1934 UNITED STATES M's'rnon FOR INHIBITING THEoxInATIoN or READILY OXIDIZABLE METALS Hans A. R-eimers, Midland, Mich.,assignor to The Dow Chemical Company, Midland, Mich., a corporzition ofMichigan No Drawing. Application June 17, 1932,

Serial No. 617,876

14 Claims.

The present invention relates to methodsi for inhibiting the oxidationof readily oxidizable metals, particularly magnesium, while maintainedabove the melting point thereof.

The term oxidation is used herein and in the appended claims to mean notonly the reaction of oxygen with a readily oxidizable metal but also thechemical reaction of such metal with nitrogen, and/or carbon monoxide,carbon dioxide, sulphur dioxide, other gases which may be constituentsof the atmosphere Whereunder such metals arelmaintained in the moltencondition. The term magnesium" is used herein and in the appended claimsto mean not only 15 elemental magnesium:;but also alloys thereof whereinthe magnesium content predominates.

It is well known that the suriace of metals such as magnesiiim,aluminum, beryllium, etc., is oxidized readily by contact with gases ofthe atmosphere, particularly at temperatures above the melting pointthereof. This fact renders difficult the operations of melting,alloying, transferring -molten metal as by ladling, etc..therebycreating a problem for which numerous solutions have beenproposed. For instance, it has been suggested to use a flux incombination with'sulphur for inhibiting oxidation of the surfaces ofmagnesium during the foregoing operations. This suggestion hasbeenreduced to practice with a fair degree of success, however, nu-

merous difficulties inhere to such practice. For example, when makingcastings by ladling or otherwise transferring the molten metal from amelting pot in which a flux is used to protect the magnesium, some fluxmay become entrained with the metal and thereby produce castingscontaining flaws due to flux inclusions, unless especial care isexercised and suitable apparatus be employed to insure a completeseparation of thefiux from the magnesium before casting.

Thus, it has been found advantageous in the case ofoperating moltenbaths of such readily oxidizable metals, particularly in connection withdie casting machines, to employ certain gases rather than a liquid flux,in order tov inhibit oxidation of the metal by gases of the atmosphere,whereby the occurrence of flux inclusions in the castings is precluded."Accordingly, it has been proposed to displace the ordinary atmospherein contact with the metals by a gas such as nitrogen, jcarbondioxidefsulphur dioxide, etc. -"Simi1ar atmospheres have" been proposedfor protecting the metal from oxidation during the melting, alloying,etc, thereof. However, in the case of magnesium, such gases-can (ous-17)not be relied upon alone to produce a protective fatmosphere, due to thefact'that the magnesium will react therewith at temperatures above the,melting point such as those encountered in the foregoing operations,whereby oxidation products are formed which may contaminate the metal.

It is therefore among the objects of the invention to provide a methodwhereby detrimental attack by atmospheric gases in contact with thesurfaces of readily oxidizable metals while maintained in the moltencondition is substantially inhibited; and further, to provide a. methodwherein suitable materials are employed so that contamination of theso-protected metal by the protective material is prevented. The presentinvention is not concerned with, nor do the claims .cover, any step inwhich the inhibition of detrimental attack by atmospheric gases uponmagnesium takes place within a casting mold. I have now .found that theoxidation of readily oxidizable metals while maintained in the moltencondition can be substantially inhibited by maintaining in theatmosphere in contact with the exposed surface thereof an oxidationinhibiting gas containing fluorine, either in ele} 30,

mental or combined form. v

To the accomplishment of the foregoing and related ends, the invention,then, consists of*,the method hereinafter fully described and particu-'larly pointed out in the claims, the following 55 description settingforth in detail several modes of carrying out the invention, suchdisclosed modes illustrating, however, but several of various ways inwhich the principle of the invention may be used. l 9

The hereinbefore mentioned protective gaseous mixture containingfluorine, which is to be maintained in contact with the exposed surfaceof the readily oxidizable metal, may contain the fluorine as: (1) theelemental gas, or (2) combined with another element or elements to-forma compound having a boiling, sublimation, or decomposition point belowabout 750 0., preferably below about 400 0.; either with or without thepresence of a diluent gas or gases, such as air.

Among the elements which may be employed in the preparation offluorine-containing compounds suitable for use in practicing my,inven-'tion are the following: antimony, arsenic, bismuth, boron, bromine,carbon,:chlorine,-hydrogen, iodine, nitrogen, oxygen, phosphorous,silicon, sulphur, tin and titanium.

The following table shows somegroups of compounds which are illustrativeof the type of compounds produced by combinations of the foreup goingelements with fluorine, which compounds I have found can be utilized increating a protective atmosphere for inhibiting the oxidation of readilyoxidizable metals. The table shows combinations of fluorine with one,two, three,-and four other elements, respectively.

The selection of the proper compound to be utilized in the production ofa protective atmosphere for use during the effecting of a particularmelting, alloying, or similar operation will depend principally upon thetemperature above which the metal is maintained during such operation.Thus; the compound selected should have a boiling, sublimation, ordecomposition point below the temperature at which the metal is to bemaintained in any particular operation.

Beyond the foregoing stipulation the points ofcost, toxicity, ease ofapplication, etc. will largely govern the selection in any case.

The application of the compound to the exposed surface of the metal maybe made in various ways. For instance, in a melting operation carriedout in a closed melting pot, if the compound selected is a solid at roomtemperatures, a protective atmosphere can be formed over the surface ofthe metal by dropping, dusting, or otherwise placing at suitableintervals of time, small quantities of the solid substance upon thesurface thereof, or upon a hot surface immediately out of contact withthe metal; or by subliming the substance with or without a diluent gassuch as air, nitrogen, etc., and passing the so-formed gas over thesurface of the metal to be protected. In the event that the compoundselected is gaseous at ordinary temperatures it can be stored incontainers, underpressure if desired, and passed withor without adiluent gas over the surface of the metal to be protected. Intransferring molten magnesium in a ladle from a melting pot to a moldinto which it is to be poured, it has been found sufficient to place one.or more small cakes or lumps of one of the comwhile it is beingtransported to and poured into the mold.

Other suitable methods of application will readily appear to thoseskilled in the art of foundry practice.

.As an example of the results which may be obtained in practicing myinvention, I will now describe the protection of magnesium fromoxidation during a melting operation, employing various compounds forproducing the protective atmosphere.

The apparatus employed consisted of an oilflred, tilting-type, coveredmelting pot holding on an average about 250 pounds of molten mag-'nesium. The pot was equipped with a mechanical feeding device whichpermitted between about .5 and 2 grams per minute of a solidfluorinecontaining compound to fall upon the surface of 1600 pounds ofmagnesium was melted and poured from the potat various intervals of timein about 150 pound lots. A total quantity of 675 grams (24 ounces) ofammonium borofiuoride (NI-I4BF4) was used in protecting the aforesaid1600 pounds of magnesium. No appreciable burning or oxidation of themetal was observed during the entire melting and pouring operation.Normal ammonium fluoride (NHlF) was substituted for the borofiuoridecompound with satisfactory results, as was the commercial ammoniumfluoride, i. e. ammonium bi-fluoride kNHiFKHF). Similar protection tothe metal was obtained by passing boron trifluoride (3P3) gas thereover,and a successful run was made using silicon -tetrafluoride (SiF4) gas asaprotective atmosphere.

Dichlorodifluoromethane (CC12F2) was also used with good results, as washydrogen fluoride (HF). Various other similar compounds, e. g. ammoniumfluophosphate, were tested with excellent results.

To illustrate further the superior effectiveness of my method ofinhibiting the oxidation of magnesium the following experiment wasperformed:-

About 175 pounds of a magnesium alloy containing 93.7 per cent by weightof magnesium. 6.0 per cent of aluminum, and 0.3 per cent of manganesewas melted in the hereinbefore described melting pot and maintained inthe molten condition at a temperature between about 700 and 760 C.(1292-l400 F.) for 15 hours. Ammonium borofiuoride was fed on to thesurface of the metal throughout the 15 hour period at the rate of about1 gram per minute. In all, 900 grams (2 pounds) of the fluorine compoundwas ,used. During the entire period a very satisfacwhich may be utilizedin carrying out my inven tion I prefer to employ one or more of thefollowing" compounds:--ammonium borofiuoride, ammonium silicofluoride,ammonium bi-fluoride and ammonium fluophosphate, or the gases evolvedtherefrom upon heating thereof. All of the foregoing compounds have aboiling, sublimation, or decomposition point below 400 C., and the,gasesevolved therefrom upon heating can be readily commingled with theatmosphere in contact with the surfaces of magnesium at elevatedtemperatures thereby inhibiting oxidation thereof.

A feature of the invention lies in the fact that a relatively smallconcentration of fluorine, or fluorine-containing gases evolved from afluorine compound, in the air or other atmosphere in contact with thesurface of molten magnesium sufflces to inhibit oxidation of the metaland to prevent ignition thereof. In other words, the action of theprotective compound is not to produce a blanketing layer of gas incontact with the surface of the molten metal which excludes oxidizinggases from contact therewith, but merely to provide a low concentrationof the protective fluorine compound in the normal atmosphere surroundingthe metal, the presence of which serves to inhibit oxidation of themetal by the gases of the normal atmosphere.

Throughout the specification and in the appended claims the term moltencondition as applied to the condition of a metal, means that thetemperature of the metal is maintained above the melting point thereofin the range normally encountered in the foundry operations of melting,alloying, etc., e. g. for magnesium the foregoing range is between about600 and 900 C. 1112"- 1650 F. l

My improved method for inhibiting the oxidation of magnesium and otherreadily oxidizable metals while maintained in the molten condition. byadding a relatively small amount of a fluorine compound to the containerfor the molten metal. has the following advantagesz-U) fullysatisfactory protection from detrimental attack by gases of theatmosphere is obtained; (2) possible inclusions of the protectivecompound in the metal are prevented; 3) the ease with which theprotective atmosphere can be produced and apn.- plied to the metal; and,(4) the low cost incident to protecting such metals from oxidation.

Other modes of applying the principle of my invention may be employedinstead of those explainedchange being made as regards the method hereindisclosed, provided the means stated by any of the following claims orthe equivalent of such stated means be employed.

I therefore particularly point out and distinctly claim as myinvention:-

1. The method of inhibiting the oxidation of molten magnesium whichcomprises maintaining a fluorine-containing atmosphere in contact withthe surfaces of said magnesium.

2. The method of inhibiting the oxidation of molten magnesium whichcomprises maintaining in the atmosphere in contact with the surface ofsaid magnesium a compound consisting of fluorine with one or moreelements from the group consisting of antimony, arsenic, bismuth. boron.bromine, carbon, chlorine, hydrogen, iodine, nitrogen, oxygen,phosphorous, silicon, sulphur, tin and titanium.

3. In melting magnesium, the step which consists in maintaining afluorine-containing atmosphere in the container wherein said magnesiumis being melted.

4. The method of inhibiting the oxidation of molten magnesium whichcomprises heating a fluorine-containing compound having a boiling,sublimation, or decomposition point below 750 C. to produce afluorine-containing gas, and commingling the gas evolved therefrom withthe atmosphere in contact with the said magnesium.

5. The method of inhibiting the oxidation of molten magnesium whichcomprises heating a fluorine-containing compound having a boiling,sublimation, or decomposition point below 400 C. to produce afluorine-containing gas, and commingling the gas evolved therefrom withthe at- ,35 mosphere in contact with the said magnesium. 6. In a methodof inhibiting the oxidation of molten magnesium, in a containertherefor, the step which consists in adding to said container arelatively small amount of a fluorine compound capable of evolving afluorine-containing gas at the temperature of the molten metal.

7. In a method of inhibiting the oxidation of magnesium, whilemaintained in the molten condition in a container therefor, the stepwhich consists in adding to said container successive small portions ofa compound selected from the group consisting ofammonium borofluoride,ammonium silicofiuoride. ammonium bi-fluoride and ammo niumfluophosphate.

8. The method of inhibiting the oxidation of magnesium during thehandling thereof in the molten condition which consists in placing uponthe surface of the said molten magnesium a compound selected from thegroup consisting of am monium borofiuoride, ammonium silicofiuoride,ammonium bi-fiuoride and ammonium fluophosphate.

9. The method of inhibiting the oxidation of magnesium while maintainedin the molten condition which comprises so placing a compound selectedfrom the group consisting of ammonium borofluoridc, ammoniumsilicofluoride. animonium bi-fiuoride and ammonium fiuophosphate, thatsaid compound is heated by the said magnesium and the gases evolvedtherefrom commingled with the atmosphere in contact with105 the surfacesof said magnesium.

10. The method which comprises melting magnesium in a containertherefor, adding to said container successive portions of a fluorinecompound having a boiling, sublimation, or decomposition point below 4000., transferring the molten metal from the container to a ladlecontaining a small amount of the said fluorine compound, and pouring themolten metal from the ladle.

11. The method which comprises melting magnesium in a container thereforand adding to said container a relatively small amount of a fluorinecompound having a boiling. sublimation, or decomposition point below 400C.

12. The method which comprises melting magnesium in-a container thereforand adding to said container a relatively small amount of a fluorinecompound selected from the group consisting of ammonium borofluoride,ammonium silicofiuoride, ammonium bi-fluoride and ammoniumfiuophosphate.

13. In a method of transferring molten magnesiurnin a ladle the stepwhich consists in adding to said ladle a relatively small amount of afluorine compound having a boiling. sublimation, or decomposition pointbelow 400 C.

14. The method which comprises transferring molten magnesium to a ladle.adding to said ladle a relatively small amount of a fluorine compound135 selected from the group consisting of ammonium borofluoride,ammonium silicofiuoride, ammonium bi-fluoride and ammoniumfiuophosphate, and pouring the molten metal from the ladle.

HANS A. REIMERS.

